The present invention relates generally to a cryocooler-cooled superconductive magnet, and more particularly to such a magnet having a superconducting lead assembly which is flexibly, dielectrically, and thermally connected to the first and second stages of the cryocooler coldhead.
Superconducting magnets may be used for various purposes, such as to generate a uniform magnetic field as part of a magnetic resonance imaging (MRI) diagnostic system. MRI systems employing superconductive magnets are used in various fields such as medical diagnostics. Known designs include cryocooler-cooled superconductive magnets wherein the cryocooler coldhead has a first stage with a design temperature between generally 40 and 50 Kelvin and a second stage with a design temperature between generally 8 and 20 Kelvin. The superconducting coil assembly of the superconducting magnet has its magnet cartridge thermally connected to the coldhead's second stage. A non-superconducting lead assembly has its two non-superconducting lead wires each with one end electrically connected to an electric current source and each with the other end thermally and dielectrically connected to the coldhead's first stage. A superconducting lead assembly has its two superconducting leads each with one end flexibly, dielectrically, and thermally connected to the coldhead's first stage and with the other end flexibly, dielectrically, and thermally connected to the coldhead's second stage. Each superconducting lead is electrically connected to its corresponding non-superconducting lead at the coldhead's first stage. Known superconducting leads include DBCO (Dysprosium Barium Copper Oxide), YBCO (Yttrium Barium Copper Oxide), and BSCCO (Bismuth Strontium Calcium Copper Oxide) superconducting leads. A superconducting lead would have its cross-sectional area large enough such that at the design current, the superconducting lead's current density would be lower than the critical current density of the superconducting lead material at a temperature equal to the coldhead's first stage design temperature and for the stray magnetic field strength it would experience from the superconducting magnet.
It is known that cryocooler performance may degrade over time. The resulting increase in temperature of the second stage will quench the superconducting wire of the superconducting coil assembly, and the resulting increase in temperature of the first stage will quench the superconducting leads of the superconducting lead assembly. Upon quenching (i.e., loss of superconductivity), the design current thereafter will flow in a non-superconducting manner in the magnet and will generate resistive heating that will destroy the superconducting wire of the superconducting coil assembly and the superconducting leads of the superconducting lead assembly. It is known to protect the superconducting wire of the superconducting coil assembly by adding a copper stabilizer wire in parallel with the superconducting wire such that, upon quenching, the current will flow through the stabilizer wire and not destroy (i.e., burnout) the superconducting wire. Simply adding a copper stabilizer wire to the superconducting leads of the superconducting lead assembly to prevent their destruction upon quenching is not a solution because of the unacceptable heat conduction that would occur in the superconducting mode along the stabilizer wire from its connections to the first and second stages of the cryocooler coldhead.
Until Applicants' invention, it was not considered possible to operate a cryocooler-cooled superconducting magnet with superconducting leads connected between the first and second stages of the cryocooler coldhead without risking the destruction (i.e., burnout) of the superconducting leads in the event of a lead quench.
What is needed is a superconducting lead assembly for a cryocooler-cooled superconducting magnet that will not be destroyed by resistive heating in the event of a lead quench.